1. Field of the Invention
The present invention relates to a boot for a universal joint which is made of a high-molecular elastomer such as rubber, in which bellows having three or more ridges are extended between a larger and a smaller ring portion, and in which a valley is formed between the first (which is numbered from the larger side, as will be common hereinafter) ridge of the bellows and the larger ring portion, and, more particularly, to a boot suitable for a constant-velocity type universal joint for connecting a propeller shaft and an axle of an automobile.
2. Description of the Prior Art
The boot for the constant-velocity type universal joint of the prior art is constructed, as shown is FIG. 1, such that a first ridge 4 of bellows 3 extending between a larger ring portion 1 and a smaller ring portion 2 is connected directly to the larger ring portion 1. In accordance with the recent speedup of the automobile and the enlargement of the compartment of a small-sized car, however, there is a tendency that the engine room becomes narrower and narrower. This makes necessary a joint boot which has the first ridge of a smaller diameter, i.e., a smaller space (including the deflectivity), and brings into the market the boot of the type in which a valley 15 is formed between a a larger ring portion 11 and a first ridge 14 of bellows 13 extending between the larger ring portion 11 and a smaller ring portion 12. In the shown example, reference numerals 21 and 23 indicate a propeller shaft and a universal joint, respectively.
It has been found that a boot of the type shown in FIG. 2 will probably have its bellows damaged at an earlier stage to permit the leakage of grease earlier than the boot of the type shown in FIG. 1 so that it has a problem in durability. This problem is deduced by our various examinations to come from the following phenomena. One is that the ridges have their pitches shortened more, because of the increase in the valleys by one, so that they are more liable to wear due to their mutual contacts--especially, the crest of the first ridge 14 is sandwiched between the shoulder 11a of the larger ring portion and the crest of a second ridge 16. Another is that the outer portion (valley) or inner portion (ridge) of the bent portion (including the ridge and valley) at the extended side is highly strained--especially at the first ridge 14 at the larger side--so that its bending fatigue is liable to be promoted, because the bellows become relatively difficult to bulge highly--the diameters of the valleys are limited so as to avoid interference with the propeller shaft.